This invention relates to ultraviolet radiation curable organic vehicles which are incorporated into compositions to provide thick film ceramic colors, which then can be cured almost instantly by ultraviolet radiation. Cured coatings of such compositions on heat resistant substrates, such as glass, metals and ceramics, can be fired at a later date, cleanly removing the organic portion and leaving a glassy pigmented ceramic enamel adhered firmly to the substrate.
Conventional coatings involving ceramic glass enamels or glazes use vehicles which are frequently oily in nature and require drying prior to application of a second color coat. Infrared heat is often used, but this is slow, costly, and energy consumption is very high. Conventional coatings also contain large percentages of solvents, which must be evaporated during the drying operation, but this is environmentally undesirable and leads to on-site safety hazards.
A more recent development is disclosed in Scheve, U.S. Pat. No. 4,306,012, incorporated herein by reference, in which a vehicle consisting of a solvent-free combination of conventional acrylate- or methacrylate-terminated prepolymers and acrylate- or methacrylate-monomers is stabilized by addition of free-radical polymerization inhibitors and mixed with a ceramic frit, then cured on a heat-resistant substrate by electron beam radiation or--after adding photoinitiators--by ultraviolet radiation. If, in practice, however, such compositions are applied by screen printing techniques, difficulty is encountered in printing because the viscosity of solvent-free media is difficult to adjust and the films tend to be too thin or to develop pin holes on firing. There are also observed difficulties, especially when using ultraviolet radiation, in curing the vehicle all the way through, if thick, highly pigmented films are laid down. Finally, without careful attention to selection of the components, difficulties are encountered in overprinting, that is, printing one layer and curing it, then printing another over it and curing it, and, finally, firing to produce a composite.
It has now been discovered that an improved ceramic vehicle can be provided, which is amenable to use in a very low solvent containing screen printable composition with ceramic frits. The vehicle employs a balance in terms of cross-link density of an acrylate- or methacrylate-terminated oligomer component, a balance in terms of functionality of an acrylate- or methacrylate-terminated monomer component, a balanced combination in terms of depth of cure of photoinitiators, and an inhibitor against polymerization while the vehicle is stored in the dark. In preferred embodiments, the new vehicles of this invention will also include flow promoters. Also provided are printable compositions comprising the new vehicles, a conventional amount of a ceramic frit (pigment), and a small, effective amount of a solvent to provide screen printability. In addition, methods are provided to print heat-resistant substrates with compositions containing the new vehicle. With the present invention, very high cure speeds are possible, up to 100 ft./min., with some colors, and this permits reduced energy costs, improved handling, and stacking. Highly pigmented coatings approaching two mils wet film thickness can be used, because of the excellent depth of cure provided by the photoinitator combination employed herein. In contrast with conventional vehicles, controlled burnout of the vehicles of this invention more readily provides pinhole-free coatings after firing. In addition, the present compositions can be used with different colors, such as blacks and whites, without modification of the photoinitiator or base vehicle, and they also lend themselves to multiple-color applications.